Burner and method of burning gas in a furnace

Abstract

A gas burner for use in a furnace and a method of burning gas in a furnace, especially but not exclusively a process furnace used in an oil cracking or refining process. The gas burner comprises two passageways with adjacent outlets. The first passage way is in fluid communication with a source of pressurized fuel gas and has an aperture through which recirculated flue gas can enter the first passageway and the second passageway is in fluid communication with a source of air. In operation, fuel gas is injected into the first passageway and recirculated flue gas is thereby drawn into the first passageway so that it mixes with the fuel gas. Fuel gas is partially combusted and a mixture of partially combusted fuel gas and recirculated flue gas flows up the first passageway and comes into contact with air from the second passageway and combusts. The use of recirculated flue gas keeps down the level of NOx emissions and as the recirculated flue gas is sucked into the first passageway by the pressurised fuel gas flow it is not necessary to provide complex pumping mechanisms.

1. A combustion furnace comprising:
 a hollow interior which, during a normal operation of the furnace, is at a desired operating temperature; 
 a floor at the bottom of the hollow interior; 
 a source of pressured fuel gas; 
 a source of combustion air; 
 a gas burner adjacent the floor for combusting the fuel gas and combustion air, the gas burner including
 (a) a first passageway having
 a fuel gas outlet therein in fluid communication with the source of pressured fuel gas whereby the fuel gas required during normal operation of the furnace is injected from the fuel gas outlet and into the first passageway, 
 an aperture disposed within the hollow interior and adjacent the floor and through which furnace gas from the hollow interior enters the first passageway and mixes with the injected fuel gas to form a mixture thereof, and 
 a first outlet through which the mixture of furnace gas and fuel gas exits the first passageway, and 
 
 (b) a second passageway separate from the first passageway having
 an inlet in fluid communication only with the source of combustion air during the normal operation so that only combustion air enters the second passageway during the normal operation, and 
 a second outlet adjacent but separate from the first outlet, from which only combustion air exits the second passageway during the normal operation; and 
 
 
 a combustion zone, located completely downstream of the gas burner and in the hollow interior, the combustion zone providing, during the normal operation and downstream of the gas burner, an initial mixing of the mixture of furnace gas and fuel gas exiting from the first outlet of the first passageway with only combustion air exiting from the second outlet of the second passageway so that a flame is produced by the combustion thereof only at a location downstream of the gas burner. 
 
     
     
       2. A combustion furnace according to  claim 1 , wherein the aperture for the furnace gas in the first passageway is provided in an exposed side wall of the first passageway. 
     
     
       3. A combustion furnace according to  claim 1 , wherein the aperture in the first passageway is located upstream of the fuel gas outlet in the first passageway. 
     
     
       4. A combustion furnace according to  claim 1 , wherein the first passageway narrows down to form a tapered portion downstream of the fuel gas outlet in the first passageway. 
     
     
       5. A combustion furnace according to  claim 4 , wherein the tapered portion of the first passageway forms a venturi. 
     
     
       6. A combustion furnace according to  claim 5 , wherein the venturi comprises a narrowing portion in which the cross sectional area of the first passageway narrows, and, downstream of the narrowing portion, a constricted portion in which the cross sectional area remains constant and is smaller than the cross sectional area of the first passageway upstream of the venturi. 
     
     
       7. A combustion furnace according to  claim 6 , wherein the first outlet of the first passageway is at the end of the constricted portion. 
     
     
       8. A combustion furnace according to  claim 4 , wherein the aperture in the first passageway is located in the vicinity of the tapered portion of the first passageway. 
     
     
       9. A combustion furnace according to  claim 1 , wherein the first gas outlet is a gas injector tube which extends into the first passageway of the burner. 
     
     
       10. A combustion furnace according to  claim 1 , wherein the first passageway is one of a plurality of first passageways for mixing fuel gas and furnace gas, each of the plurality of first passageways having an aperture for gas from the hollow interior of the furnace, being in fluid communication with a source of pressurized fuel gas, and having a first outlet adjacent to the second outlet of the second passageway. 
     
     
       11. A combustion furnace according to  claim 1 , wherein walls form the second passageway and the walls are arranged in an octagonal structure. 
     
     
       12. A combustion furnace according to  claim 1 , further including a pilot gas outlet in the second passageway which is connected to a fuel supply, the pilot gas outlet (i) being turned off during normal operation of the furnace, and (ii) being turned on only to effect a preheating of the second passageway. 
     
     
       13. A combustion furnace according to  claim 1 , wherein the first passageway, the first outlet, the second passageway, the second outlet are substantially vertically oriented whereby the fuel gas vertically exits the first passageway, the combustion air vertically exits the second passageway, and the combustion zone is thus located completely vertically downstream of the gas burner.